The invention relates to a fuel injector for a combustion engine, in particular in a common rail injection system in which the injector is continuously stressed by fuel held in a preliminary reservoir under high pressure, whereby the fuel injector contains a nozzle needle linked with an actuating element that serves to control a nozzle-opening process with a needle tip and a valve seat that works together with the needle tip of the nozzle needle, whereby the valve seat has a conical sealing surface with the opening angle a1 and the needle tip, when the injector is closed, has a conical sealing surface contacting it with an opening angle a2 and needle tip and needle housing delimit a ring-shaped flow channel.
In fuel injectors of this type, there is the difficulty that often even after a short period of service a great deal of damage can be found because of cavitation below the valve seat on nozzle needle tip and needle housing which can lead to a lack of seal integrity in the valve seat and to failure of the injector. This type of cavitation damage can occur in particular in common rail injector systems since because of the continuous presence of high pressure from the fuel under high pressure in the preliminary reservoir, considerably longer cavitation phases can occur during opening and closing of the injector compared to injectors of standard fuel injection systems.
A fuel injector for combustion engines is known from DE 36 05 082 A1 which is intended for a standard injection system and in which the needle tip sealing surface that works together with the valve seat is provided with a ring groove which serves to create a turbulent interface in the flows. Downstream of the sealing surface, a convex-shaped shroud surface section is provided on the needle tip which is followed by a concave-shaped shroud surface section. Because of this, the tendency of the flow to break down in the area of the valve passage will be decreased and the flow and/or the spray pattern will be improved.
DE 196 34 933 A1 shows a fuel injector in which two areas having different cone angles have a valve sealing surface in the transition, downstream of the valve seat, have a groove-shaped expansion. The expansion serves to increase the metering accuracy of the injection quantity, in that a defined position is created with respect to the sealing edge. DE 195 47 423 A1 also uses groove-shaped expansions below the valve seat in the nozzle body or nozzle needle in order to produce a defined line of contact. The radial recess that represents an expansion is very flat and specified at 0.01 to 0.06 mm. It should be assumed that this expansion increases the cavitation still further and the nozzle needle and the nozzle body are subject to damage by erosion in the adjacent walls which ultimately leads to injector damage.
The task of the invention is to produce a fuel injector which has low susceptibility to cavitation damage on nozzle needle and needle housing in the area of the valve seat.
This task is solved by the fuel injector indicated in claim 1. Advantageous further developments of the injector according to the invention are identified in the subclaims.
The invention produces a fuel injector for a combustion engine, in particular of a common rail injection system, in which the injector is continuously stressed by fuel held under high pressure in a preliminary reservoir. The fuel injector contains a nozzle needle linked with an actuating element that serves to control a nozzle-opening process with a needle tip and a valve seat that works together with the needle tip of the nozzle needle, whereby the valve seat has a conical sealing surface with the opening angle a1 and the needle tip, when the injector is closed, has a conical sealing surface contacting it with an opening angle a2 and needle tip and needle housing delimit a ring-shaped flow channel. It is provided that the opening angle a2 of the needle tip sealing surface is smaller than the opening angle a1 of the sealing surface of the valve seat and that in the flow direction of the fuel, following the sealing surfaces, an expansion of the ring-shaped flow channel is formed between needle tip and needle housing in such a way that the expansion is designed as a cavitation space, in which the implosion of the cavitation bubbles will occur away from the wall. This means that size and shape of the cavitation space is maintained with the goal that the cavitation bubbles are guided at a distance from the walls of the ring-shaped flow channel formed by the nozzle needle and needle housing and thereby no erosion on the walls occurs.
Because of the inverse seat angle difference, the narrowest part of the needle sealing seat is located at the downstream end where cavitation forms as defined, its cavitation bubbles then do not have any opportunity in the following expansion to deposit themselves on the walls of needle tip and/or needle housing and thus cannot cause any damage. Since cavitation on the nozzle needle is more critical than on the needle housing, it can be adequate to make the expansion preferably in such a way that cavitation bubbles disintegrate at least far from the walls of the nozzle needle. In fact, erosion on the nozzle needle influences the function of the injector by changing the opening behavior.
According to an especially preferred embodiment, it is provided that the expansion of the flow channel is provided between needle tip and needle housing directly on the sealing surfaces of needle tip and valve seat.
Preferably the expansion of the flow channel between needle tip and needle housing is formed by a curve that is concave in cross section of at least one of the surfaces of needle tip and needle housing.
An especially advantageous embodiment of this provides that the concave curve of the surface of the needle tip and/or needle housing is formed by a radius.
According to a preferred embodiment, it is provided that the concave curve of the surface at the upstream side gradually changes, with one edge, into the sealing surface of needle tip and/or valve seat.
In addition, it can be provided that the concave curve of the surface at the downstream side gradually changes, with one edge, into the surface of needle tip and/or needle housing.
According to an especially preferred embodiment of the fuel injector according to the invention, it is provided that both on the surface of the needle tip and needle housing an expansion of the flow channel with concave curve is provided and that the center of the expansion of the needle housing is displaced toward the upstream direction compared to the center of the needle tip expansion when the valve is closed.
According to a preferred embodiment, it is provided that the expansions on needle housing and needle tip are formed by equal radii.
The opening angle a1 of the valve seat is preferably between 50xc2x0 and 60xc2x0, preferably between 55xc2x0 and 65xc2x0.
According to an especially preferred embodiment, the opening angle of the valve seat is around 60xc2x0.
The opening angle a2 of the needle tip sealing surface is advantageously between 0.5xc2x0 to 3xc2x0, preferably 1xc2x0 and 2xc2x0, smaller than the opening angle a1 of the valve seat.
It is especially advantageous to make the opening angle a2 of the needle tip sealing surface 1.5xc2x0 smaller than the opening angle a1 of the valve seat.
According to an advantageous further development of the invention, it is provided that on the needle tip, upstream of the sealing surface, a transition surface is formed that has an angle a3 between that of the needle body and that of the sealing surface of the nozzle needle. This transition surface improves the flow behavior at the transition from needle body to sealing surface.
This transition surface is preferably formed by a conical surface.
Preferably the transition surface is designed in such a way that it approximately halves the angle between the sealing surface of the nozzle needle and the needle body.
According to a preferred further development of the fuel injector according to the invention, it is provided that the needle tip has an end section that comes to a point. This has the advantage that the nozzle needle extends with its end section far into a hole formed on the downstream end of the needle housing which decreases the pocket hole volume.
Preferably the end section that comes to a point is formed as a cone.
According to a preferred embodiment, it is planned that the cone forming the end section that comes to a point has an opening angle a4 that is smaller than the opening angle a2 of the needle tip sealing surface.
The opening angle a4 of the end section is advantageously between 40xc2x0 and 65xc2x0, preferably between 50xc2x0 and 55xc2x0.
According to a further development of the invention, it is provided that the needle tip, upstream of the sealing surface, has a beadshaped section that is enlarged compared to the diameter of the needle body.
This bead-shaped section can be formed of successive conical and/or cylindrical ring surfaces.
Alternatively, the bead-shaped section can be formed of a lens-shaped or ball-shaped surface.
Preferably the diameter of the bead-shaped section is 1.05 times to 1.2 times, preferably 1.1 times to 1.15 times, the diameter of the needle body of the nozzle needle.
The longitudinal expansion of the bead-shaped section in the direction of the needle axis is advantageously 0.2 times to 0.6 times, preferably 0.25 times to 0.35, times the diameter of the needle body of the nozzle needle.
In the following, preferred embodiments of the invention will be explained using the drawing.
FIG. 1 shows a cut-away side view of a fuel injector in the area of the needle tip according to a first embodiment of the invention;
FIG. 2 shows a cut-away side view of a fuel injector in the area of the needle tip according to a second embodiment of the invention;
FIG. 3 shows a diagram of the needle tip of the nozzle needle of the first embodiment shown in FIG. 1 with detail X enlarged;
FIG. 4 shows a diagram of the needle tip of the nozzle needle of the second embodiment of FIG. 2 in enlarged scale;
FIG. 5 shows the needle tip of a nozzle needle according to a third embodiment of the invention with details X and Y.